This invention relates to a process for producing highly active denitrating catalysts, and more particularly to a process for producing catalysts useful in a reaction in which nitrogen oxides are selectively catalytically reduced with ammonia (NH.sub.3).
It is recognized that the nitrogen oxides (hereinafter referred to as "NO.sub.x ") released to the atmosphere from thermoelectric power stations, ironworks, chemical plants, motor vehicles, etc., cause photochemical smog, so that much attention has been given to the treatment of NO.sub.x in recent years.
Methods of removing NO.sub.x heretofore proposed include one in which NO.sub.x are catalytically reduced with the use of NH.sub.3 as a reducing agent. This method appears more advantageous than others since NH.sub.3 selectively reacts with NO.sub.x as represented by the following equations, even when the exhaust gas contains more than 1% by volume of oxygen. EQU 6NO+4NH.sub.3 .fwdarw.5N.sub.2 +6H.sub.2 O EQU 6NO.sub.2 +8NH.sub.3 .fwdarw.7N.sub.2 +12H.sub.2 O
The known catalysts useful in this method include those comprising a heavy metal compound supported on a carrier such as active alumina, silica alumina or zeolite. These catalysts are generally intended for use in fixed beds. Accordingly if it is attempted to achieve an increased space time yield (S.T.Y.), the emission will involve an increased pressure loss, while solid particles in the emission will clog up the catalyst layer. To overcome such drawbacks, it has been proposed to use carriers of porous structure.
Generally thermoelectric power stations and ironworks releases tremendous quantities of emissions which require large quantities of catalysts for treatment. Catalysts composed of porous carriers for this purpose therefore should preferably be large-sized and have high strength so as not to be fractured during charging. A catalyst is already known which fulfills such requirements and which is prepared by converting the surface layer of steel material to an aluminum alloy, immersing the resulting steel material in an aqueous alkali solution to dissolve out the aluminum and to thereby render the surface layer porous, and subjecting the resulting material to suitable oxidizing treatment. This catalyst is practically useful because the porous surface layer remains relatively intact and acts effectively even in the presence of SO.sub.2 or water vapor contained in the emission. However, the catalyst has the drawback that it exhibits seriously reduced denitrating activity at about 300.degree. C. and slighter lower temperatures.